The present invention relates to satellite communications. In particular, the present invention relates to reduction of latency in a satellite communications network.
Communication satellites routinely relay staggering amounts of information between multiple earth terminals every day. Uplink signals and downlink signals transmitted through a communication satellite, however, are subject to delays corresponding to the signal time-of-flight through space and the atmosphere between the earth terminals and the satellite. In other words, electromagnetic propagation (the signal) travels at the speed of light (as modified by whatever medium it travels through, including the atmosphere). The time-of-flight introduces a time delay or latency in the signal corresponding to the amount of time it takes the signal to reach its destination. Furthermore, other significant contributors to overall latency typically exist within a system, including, for example, data processing time delay.
Geostationary orbit satellites are often used for communications because of their unique orbital property of staying fixed at a given longitude, zero degrees latitude, and a constant range. That is, a geostationary satellite remains at a fixed (earth-centered) location in the sky (i.e., the satellite has a non-varying azimuth and elevation). However, geostationary satellites must fly in a singular, circular orbit having a zero inclination and an altitude of approximately 22,000 miles. At this relatively great distance, the latency for one "leg" (from the Earth to the satellite, or vice versa) of signal transfer is approximately 1/8 of a second. While this may not appear to be a significant delay, it can become a serious or compromising aspect for real-time applications when (as is typically the case) several legs of travel are needed. One "hop" (i.e., two legs, one up, one down) takes about 1/4 second.
Thus two hops, a typical scenario for earth terminals to transmit a signal to another earth terminal (e.g., another individual user or to a central communications facility) and receive a response, can take more than 1/2 second. If individual earth terminals must pass information through a central communications station, a complete communications loop (round trip query/response) may take more than one second.
In real-time applications (including action video games, for example) a one second latency is unacceptable. Furthermore, many business applications, such as video-conferencing and work sharing (e.g., spreadsheet or whiteboard applications) are very sluggish given these latencies. Given the trend to increasing satellite communications with decreasing cost, such applications, were it not for the latencies involved, would be poised to introduce a new level of worldwide interactivity.
In the past, however, no techniques have been proposed to reduce latencies to acceptable levels. One reason is that satellite designs fall into a few general categories, none of which is particularly suited to reducing latency. A satellilte that demodulates an uplink signal and remodulates data for a downlink is referred to as "regenerative". A satellite that demodulates an uplink signal, decodes the signals, and recodes the signals is typically referred to as a "regenerative decode/recode" system or just, "decode/recode". On the other hand, a satellite which simply forwards the received uplink signals unaltered to a ground station is typically referred to as a "bent pipe" system. These past satellite designs, while useful in many situations, do not provide the satellite with anything more than the ability to relay information. Thus, data that needs to be processed in some fashion must make several hops between specialized processing earth terminals, each hop increasing the latency from original transmission to final reception.
A need exists in the industry for a method for reducing latency in satellite communication systems.